1. Field of the Invention
The present invention relates to organic electroluminescence elements, and particularly to an organic electroluminescence element used in display devices and lighting, and manufacturing method thereof.
2. Description of the Related Art
An element from which luminescence (electroluminescence) can be obtained by applying voltage to a thin film of organic material sandwiched between two electrodes is called an organic electroluminescence element (hereafter called organic EL element). An organic EL element using an organic low-molecular-weight material was discovered in the 1960's (see Non-Patent Reference 1: M. Pope et al., Journal of Chemical Physics vol. 38, pages 2042 to 2043; 1963), and the element structure and practical processes were developed in the 1980's (see Non-Patent Reference 2: C. W. Tang and S. A. Vanslyke, Applied Physics Letters vol. 51, pages 913 to 915; 1987). An organic EL element using an organic low-molecular-weight material is characterized in having few pixel defects since the organic thin film thereof is prepared by vacuum deposition in which mixing-in of impurities and dust in a process under vacuum is low. Subsequently, in the first half of the 1990's, an organic EL element using polymeric molecules was reported (see Non-Patent Reference 3: J. H. Burroughes et al., Nature vol. 347, pages 539 to 541; 1990). An organic EL element using polymeric material is characterized in that a convenient process under atmospheric pressure is used and material loss is low, since the organic thin film thereof is prepared by applying, through a wet process, a liquid solution or dispersion liquid obtained as a result of dissolving polymeric molecules in a solvent. Both organic EL elements are characterized in being self-luminous and bright, having low viewing angle dependency, facilitating area-enlargement and fine-arraying, and so on, and are being developed as a source of luminescence in a display and a light source for lighting.
FIG. 1 is a structural cross-section view of the conventional organic EL element described in Non-Patent Reference 2. An organic EL element 500 described in FIG. 1 includes a transparent substrate 501, a transparent bottom electrode 502, an organic layer 503, and an opaque top electrode 504. The transparent bottom electrode 502 is laminated on the transparent substrate 501 in a structure that allows luminescence from the organic layer 503 to be retrieved from the substrate-side. A metal electrode is used as the opaque top electrode 504, and the luminescence from the organic layer 503 is reflected. Hereinafter, an organic EL element having the same structure as the organic EL element 500 shall be described as a bottom emission organic EL element.
In contrast, FIG. 2 is a structural cross-section view of the conventional organic EL element having a structure that allows the luminescence from the organic layer to be retrieved from the top electrode-side (see Patent Reference 1: Japanese Unexamined Patent Application Publication No. 10-162959, and so on). An organic EL element 600 described in FIG. 2 includes an opaque substrate 601, an opaque bottom electrode 602, an organic layer 603, and a transparent top electrode 604. The opaque bottom electrode 602 is laminated on the opaque substrate 601 in a structure that allows luminescence from the organic layer 603 to be retrieved from the transparent top electrode 604. Hereinafter, an organic EL element having the same structure as the organic EL element 600 shall be described as a top emission organic EL element.
When considering the applicability to an active matrix-type organic EL display including an organic EL element and a thin film transistor (hereafter called TFT) driving such organic EL element, the top emission organic EL element is more suitable than the bottom emission organic EL element. This is because, since luminescence is retrieved from the substrate-side in the case of the bottom emission organic EL element, the area of the pixel area that can be occupied by an organic EL luminescence unit is limited to an area on the substrate other than that for the opaque TFT and electrical wiring. At the same time, the area for TFT and electrical wiring within a pixel needs to be minimized since securing an area for the organic EL element is prioritized, thus placing a constraint on design freedom.
In contrast, since luminescence is retrieved from the side opposite the substrate in the case of the top emission organic EL element, the organic EL element can be formed on top of the TFT layer on the substrate-side, and the area of the TFT layer can be widened as much as the pixel area. Accordingly, since it is possible to increase the amount of current supplied to the organic EL element since the TFT channel width is enlarged, or it is possible to increase the number of TFTs and form a current compensation circuit, the in-plane luminance distribution of the display becomes uniform. In addition, since the portion of the organic EL element area occupying the pixel area increases, the luminescence load per unit pixel decreases and the operational life of the display improves.
In particular, in the top emission organic EL element which is highly advantageous in applications for displays, a metallic oxide electrode such as an indium tin oxide (hereafter called ITO) electrode is used as the electrode for the transparent top electrode 604. Since it is difficult to form these into a thin film having satisfactory transparency and conductivity by resistance heating deposition, the sputtering method and other film-forming methods using plasma are used.
Furthermore, generally speaking, in the organic EL element structure, the bottom electrode is an anode, and the top electrode is a cathode. Particularly, in the case of an organic EL element using organic polymeric molecules, a polymer layer is formed by a wet process such as the spin coat process or inkjet process. Alkaline metal, alkaline earth metal, or salts thereof, used as a cathode having a function for supplying electrodes, react with water or oxygen and easily become unstable. Therefore, in the case where a cathode is the bottom electrode, the alkaline metal, alkaline earth metal, or salts thereof, making up the bottom electrode reacts with the organic layer which is a liquid layer at the initial stage of forming, and mutual elution or mutual dispersion occurs at the lamination interface, and thus control of the lamination interface becomes difficult. From this perspective as well, the structure in which a cathode is the top electrode is adopted.
As described above, in the case where the transparent top electrode represented by ITO is a cathode, an electron injecting layer having the following characteristics-conditions is needed between the transparent cathode and the organic luminescent layer, in order to inject electrons into the organic luminescent layer.
(1) Promotes electron injection from the transparent cathode to the organic luminescent layer,
(2) Protects the organic luminescent layer from damage during transparent cathode film-forming, and
(3) Optical transparency is high.
(1) is because, the transparent top electrode of a metallic oxide represented by ITO has, from its work function, excellent hole injecting characteristics but has poor electron injecting characteristics, and the electron injecting characteristics needs to be supplemented. (2) is for protecting the organic luminescent layer from plasma damage leading to the deterioration of luminescence efficiency, the rise of drive voltage, and the deterioration of the operational life of the element, since the film-forming method for the metallic oxide such as ITO is a film-forming method using plasma. (3) is for the transmission of the luminescence of the organic luminescent layer.
In order to satisfy the aforementioned requirements for an electron injection layer, conventional organic EL elements are disclosed in Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2004-127740, Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2000-260572, Patent Reference 4: Japanese Unexamined Patent Application Publication No. 2004-327436, and so on.
In Patent Reference 2, a metal-doped organic material is used as a lower layer of a transparent electrode, and with the existence of the metal-doped organic material, damage to the organic luminescent layer during the forming of the transparent cathode is reduced and electron injection is promoted. As the organic material used in the metal-doped organic material layer, a generally used π (pi) electron low-molecular-weight electron transport material has been proposed.
Furthermore, Patent Reference 3 discloses the use of metal-doped fullerenes.
Furthermore, in Patent Reference 4, electron transporting characteristics exhibiting ohmic behavior are realized by using a layer including fullerenes as an electron transport layer, and forming in the following order: a layer including an alkaline fluoride, in the upper layer or lower layer of the electron transport layer; and then an upper electrode including a conductive material.